In recent years, an optical element categorized as a photonic crystal (PC) has attracted an attention.
This is mainly owed to techniques in which a periodic structure of an optical material is formed to produce a periodic refractive index profile caused by a difference in refractive index, thereby making effective use of behavior of light in such a specific refractive index profile, or making effective use of such a phenomenon that, when the structure having a specific refractive index profile includes a light emitting material etc., its light emission state is controlled (see, E. Yablonovitch “Phys. Rev. Lett.” Vol. 58, p. 2059, 1987). Heretofore, an applicability of an optical element based on those techniques has become a controversial subject.
Regarding the optical element techniques, hitherto, a so-called distributed feedback (DFB) laser attained by exploiting a one-dimensional periodic structure to a semiconductor laser has already come into practical use. This may be called an optical element to which a one-dimensional photonic crystal is applied.
Further, in recent years, there has been an impressive development on a laser element, a so-called vertical cavity surface emitting laser (VCSEL) as well. As shown in FIG. 18, however, in this element, light-confinement in a light emitting direction, i.e. a direction vertical to a substrate surface, is effected by two multilayered film mirrors 1801 and 1802 made up of a porous compound semiconductor formed through epitaxial growth, while light-confinement in a horizontal direction with respect to the substrate surface is realized according to the principle that total reflection at the boundary between a cylindrical semiconductor and the air allows light-confinement; this is because the entire cylindrical semiconductor has a higher refractive index than its surrounding substance (air) like a core of an optical fiber.
In general, well known as a manufacturing method for the multilayered film mirror is a deposition or sputtering method. In the case of aiming at ones having a high quality, small loss, and high matching property with an active layer (light emitting material layer) like a surface emitting laser, an epitaxial growth technique capable of producing a crystal having a monolithic structure or a substantially monolithic structure over multiple layers is inevitably used.
Meanwhile, there has been known regarding the multilayered film mirror another monolithic manufacturing method aside from the epitaxial growth technique, for example, a technique disclosed in Lehman Reece et al. “Applied Physics Letters”, Vol. 81 (2002), pp. 4895. The disclosed technique is such that a silicon substrate is anodized in an HF solution, and an intensity of an electric field applied at this time is modulated periodically with respect to time, whereby two layers having different porosities are alternately formed. The silicon that has been made porous through anodization maintains its crystallinity, i.e. monolithic quality, even after turned porous since the substrate is originally made of single crystal. Also, as disclosed in the document, low-temperature anodization affords a more uniform interfacial structure between layers, so a multilayered film of optical quality can be formed.
Nowadays, the most popular one among study cases that are being reported as the “photonic crystal” is a two-dimensional photonic crystal formed by a process of patterning a two-dimensional periodic structure to a slab-shaped semiconductor etc. In particular, there are made extensive studies on the basic principle of application of the two-dimensional photonic crystal having a two-dimensional structure of cylindrical pores to optical communication parts, for example.
In such a two-dimensional photonic crystal, a light-confinement performance in one non-periodic direction (in general, in a thickness direction) may be inferior to those in the remaining two periodic directions in terms of light confinement under control. In contrast, some attempts have been made to obtain a periodic structure in all of the three directions, i.e. a so-called three-dimensional (3D) photonic crystal structure.
As for the three-dimensional photonic crystal developed so far, for example, there is an element called a double-cross or wood-pile type element, which is manufactured by a laminating method (see, Noda “Photonic Crystal Technique and its Application” p. 128, 2002, CMC publishing, Co. Ltd.). Given as another example thereof is an element manufactured by a micromechanics manufacturing method (see, Hirayama et al. “Photonic Crystal Technique and its Application” p. 157, 2002, CMC publishing, Co., Ltd.). Besides, given as still another example thereof is an element manufactured by a thin-film lamination growth method called self-cloning (Japanese Patent Publication No. 3325825 B or Sato “Photonic Crystal Technique and its Application” p. 229, 2002, CMC Publishing).